It takes 21 minutes for 9 people to paint 9 walls. How many minutes does it take 14 people to paint 14 walls?
Explanation: Imagine that each person is assigned one wall, and all 9 people begin painting at the same time. Since everyone will finish painting their assigned wall after 21 minutes, it takes one person 21 minutes to paint one wall. If we have 14 people and 14 walls, we can again assign one wall to each person. Everyone will take 21 minutes to paint their assigned wall. In other words, it takes 21 minutes for 14 people to paint 14 walls.